The invention especially concerns methods for improving the mixing treatment of a liquid and in this connection methods for improving the way in which analytes in the liquid are brought to reactants that are immobilized on an area of the support and in particular of the biochip that is occupied by the liquid.
The following problem occurs in biochip applications and especially in immunoassay applications in which binding reactions are detected between reactants that are immobilized on a support surface and analytes that are present in a liquid that wets the support surface. The binding of analytes to immobilized reactants lowers the concentration of analytes in the liquid in a boundary layer on the support surface resulting in a depletion of analytes in the sample liquid in a boundary layer. Due to the usually low analyte diffusion rate which is normally only a few μm/s or less, new analyte molecules are not resupplied rapidly enough from the sample liquid volume so that long incubation times are required for immunological tests or such like to achieve an adequate measuring effect. There are various approaches for solving this problem in the prior art.
A mixing process is described in U.S. Pat. No. 6,485,918 for a microarray biochip with a deformable cover which is placed over the surface of the microarray. Deformation of the cover generates a flow movement in the liquid between the cover and microarray surface. Although this known method is suitable for flat biochips having a reactant immobilized on the bottom, it has the disadvantage that the cover must always be in contact with the sample liquid during the mixing process. Additional processing steps such as washing steps, reagent addition etc. may necessitate an opening of the cover with the associated risk of losing sample liquid and contamination.
A method and device for mixing samples near the interface in biosensor systems, namely biochips is known from WO 00/10011. In order to increase the sensitivity the liquid in the biochip is excited in this known method by mechanical waves (sound, ultrasound or surface waves) which is intended to improve mixing of the sample liquid especially at the chip/liquid boundary layer in order to enhance the diffusion of the analyte.
A device for controlling the temperature and mixing the contents of vessels of a microtitration plate for immunological tests is known from EP 0 281 958 A2. This known device comprises a cover which defines a hollow space into which a gas line discharges. A boundary wall of the cover that faces the titration plate is provided with gas outlet openings which are arranged eccentrically relative to the vessel axes of the individual vessels of the microtitration plate and are aligned at an angle to the surfaces of the liquids in the individual vessels. Temperature control and generation of a rotary mixing movement of the liquid in the individual vessels is achieved by blowing in warm air through the gas outlet openings.
U.S. Pat. No. 6,063,564, U.S. Pat. No. 4,479,720 and U.S. Pat. No. 5,009,998 for example also concern the improved mixing of sample liquids in sample tubes as liquid containers.
The mechanical mixing processes known from the prior art such as shaking, application of ultrasound, vortexing etc. have proven to be not particularly effective and advantageous for biochips with a flat support or planar support surface with an array of reactants.